The present invention relates in general to a liquid level indicating system and more specifically to an anti-slosh circuit switchable between a short time constant and a long time constant without requiring the use of a large capacitor. The invention is particularly applicable to measuring the fuel level in a motor vehicle fuel tank and accordingly will be described with reference to such a system.
A basic fuel gauge system includes a liquid level sensor in a fuel tank and a gauge receiving a sensor signal to indicate to the driver the status of the sensor unit and, therefore, the fuel level. A typical sensor may include a float body pivoting on a float arm connected to a variable resistance which generates a voltage signal proportional to the level of the float. However, fuel level variations due to movement of the vehicle (known as "fuel slosh") cause the sensor signal to include transient errors with respect to the actual level fuel in the tank. Since these transient errors have an average value of zero, the actual fuel level can still be determined by obtaining the average of the sensor signal over a sufficiently long time period. Accordingly, a variety of damping arrangements have been incorporated into gauge systems to eliminate rapid transient movements of the fuel gauge due to fuel slosh.
Electronically damped anti-slosh circuits frequently employ RC integrator circuits to derive an average signal. It is also known to limit the slew rate of a signal amplifier to slow the response of an anti-slosh circuit in order to damp the system response to changes in the sensor signal. In order to obtain the long time constants which are required in anti-slosh circuits, the prior art has used undesirably large capacitors, such as electrolytics, which are expensive and create packaging problems. Leakage currents associated with the large capacitors also create disadvantages such as inaccuracy or nonsymmetrical operation as the level signal slews up or down.